a. Field of the Invention
The present invention relates generally to vector modulators and more particularly relates to vector modulators capable of varying the magnitude and phase of a signal that may be passing in either direction through the apparatus.
b. Description of the Background
Various methods of modulating RF signals have been developed. In particular, vector modulation in which the resultant signal is the vector sum of two amplitude modulated signals has demonstrated many advantages over other methods. These individual signals that combine to form a vector sum are phase shifted with respect to one another by a predetermined amount and may consist of a carrier signal that is modulated by a data input signal. In a typical phased array antenna system, vector modulators are employed to vary both the amplitude and phase of a signal, as opposed to merely varying the phase with a phase shifter. This type of system can reduce the amplitude of the side lobes (amplitude tapering) thereby optimizing antenna pattern. These types of antenna systems typically include a plurality of distinct antenna elements that are individually directed to transmit and/or receive a signal in a particular orientation.
Typical vector modulators, such as those utilized in conventional phased array antenna systems, characteristically employ amplifiers to adjust the signal gain and produce variability in the signal amplitude. Variable phase shifts and gains are obtained by adjusting the relative amplitudes of the vectors. The major drawback realized in utilizing amplifiers is that these devices are unidirectional. That is, they produce a different amplitude response and phase shift depending upon which direction the signal is being sent through the device and restrict the versatility of the circuit with respect to achieving continuous phase and amplitude states.
In the case of a phased array antenna system, the signal passes through the device in one direction for transmitting, and the opposite direction for receiving. Previous attempts using active phase shifters that employ a vector sum method have been subject to large power losses due to the architecture schemes of the phase shifters and inefficiencies in utilizing amplifiers.
Therefore, there is a need for a vector modulator capable of independently varying the magnitude and phase of a signal, and to accomplish this in either direction through the apparatus and allow amplitude tapering of the antenna.